Method of fabricating an encapsulated motor

ABSTRACT

A method of fabricating an electrical motor, such as a hybrid permanent magnet stepping motor or a variable reluctance motor. A rotor is assembled on a rotor shaft and includes at least one lamination stack and a pair of supporting bearings, with the outer diameter of the bearings being slightly larger than that of the lamination stack. A stator is assembled from a stator lamination stack including a pole configuration and a pair of unmachined end caps registered and secured to the lamination stack such as by thru bolts. The registered stator assembly is potted to unitize the assembly, fixing the relationship between the end caps and the lamination stack, and providing a smooth continuous bore through the center of the stator assembly. The thus potted assembly is then machined as by diamond lapping to form a continuous bore accurately machined through the center of the stator, concurrently forming bearing surfaces in the end caps and an intermediate machined section in the lamination stack. The rotor assembly is inserted into the stator assembly with the machined bore providing bearing mounting surfaces in the end caps and an air gap for rotation of the rotor in the lamination stack.

FIELD OF THE INVENTION

This invention relates to electrical motors, and more particularly tomotor structures and assembly techniques which provide relatively smallsize motors of simple and inexpensive construction.

BACKGROUND OF THE INVENTION

At the outset, it will be pointed out that the invention relates toconstructional features and assembly techniques for motors rather thanthe specific electromagnetic mechanism which drives them. As a result,the present invention can be applied to a number of different motortypes including switched reluctance motors, permanent magnet brushlessmotors and hybrid stepping motors among others.

Chai et al. U.S. Pat. No. 4,029,977 shows a relatively conventionalconstruction for a variable reluctance stepping motor. It is seen thatthe motor includes an external case machined to register with a pair ofend bells which also must be machined to register with both the case andthe rotor bearings. The external case, in turn, serves to register awound and insulated stator lamination stack. Thus, when the elements areassembled, the rotor is held in its bearings in the machined surfaces inthe end bells, the numerous registration devices assure that the rotoris properly positioned within the stator. However, such an arrangementcarries with it a degree of manufacturing expense because of all of theseparate parts must be fairly precisely machined in order for all of theregistration systems to function in properly aligning the rotor withinthe internal bore of the stator.

It has been proposed to produce a motor without an external case,exposing the exterior of the stator laminations. One such approach isillustrated in Kawada et al. U.S. Pat. No. 4,538,084, and another inKawada et al. U.S. Pat. No. 4,626,725. The former uses threaded rodswelded within the stator lamination assembly in order to appropriatelyregister the end caps to the stator. The latter uses threaded rods oftwo lengths, one interfitting into the end caps and the other on whichthe end caps bottom in order to maintain registration of the end caps tothe stator. While those approaches save the expense of the motorhousing, they add complication in the form of such elements forattaining and maintaining registration of the end caps (and thereforethe rotor) to the stator.

It is also applicants' understanding that a motor of the general typeillustrated in the aforementioned Chai et al. patent has been configuredwithout a case, that is, with the stator laminations exposedintermediate a pair of end caps. As applicants understand it, in orderto achieve the elimination of the case, the end caps were altered in twoways. First of all, tapered tabs were formed on the periphery of the endcaps, projecting toward the lamination stack at three points around eachend cap in order to accurately register the end caps with the laminationstack. Secondly, apertured mounting ears were also formed on the endcaps, protruding at right angles from the tabs. The mounting earsreceived bolts which spanned the lamination stack between end caps tofirmly secure the end caps to the lamination stack, thereby not onlyproviding register but also rigidity. In that registered rigidconfiguration, means were then provided to machine the bore whichextended through the end caps and stator assembly. A multi-stationlapping machine was used to simultaneously machine bearing surfaces inthe end caps and the stator bore. The rotor was then positioned in themachined aperture with the rotor bearings mounted in the end caps andthe rotor free to rotate within the stator.

That approach suffers from certain problems. First of all, the exteriortabs and mounting ears on the end caps require the envelope occupied bythe motor to be enlarged; in many applications where space is at apremium the enlarged size could be unacceptable. More importantly,machining the bearing surfaces and stator ID with the stator fullyassembled results in machining debris entering the motor. The motor wasconstructed of open configuration, providing large apertures in the endcaps which allowed the motor to be thoroughly washed in an effort toremove the machining debris. In addition, machining debris could betrapped within the coils, fall between the end caps and the stator orotherwise lodge itself in apertures in the stator assembly, and thatmachining debris could work loose during later operation of the motor tocause premature failure.

The stator machining problem could be particularly severe for motors ofthe "enhanced" type which have permanent magnet segments interposedbetween rotor teeth, since machining of the rotor bore would then createmachining debris which included highly magnetic particles. The magneticparticles would be extremely difficult to remove by standard washingtechniques and motor stators were sometimes varnished or otherwiseinternally coated to secure the magnetic particles which could not beremoved in position. Thus, there was created a significant cleanupproblem and the possibility of motor failure by means of unremovedmachining debris.

One of the problems which has necessitated relatively elaborateregistration devices is thermal cycling of the motor in the case wherethe end caps have a significantly different coefficient of thermalexpansion than the stator laminations. The stator laminations aretypically steel. The end caps, however, must be nonmagnetic and oftenmade of aluminum which expands much more than the steel when the motoris operating and dissipating heat, and contracts much more than thesteel when the motor is at rest and cools. If the end caps are not heldin rigid and fixed register with respect to the stator laminations, whenthe motor is cyclically heated and cooled during operation, the end capscan "walk" with respect to the stator, ultimately causing misalignmentof the rotor within the stator.

SUMMARY OF THE INVENTION

In view of the foregoing, it is a general aim of the present inventionto provide a small and relatively inexpensive motor with exposed statorlaminations which is simpler and more economical to fabricate than theprior art discussed above.

In that regard, it is an object to minimize the cost and complexity ofboth the end caps and the lamination stack while still achieveing thenecessary precision in final assembly.

Accordingly to a more detailed aspect of the invention, it is an objectto minimize pilot devices on the motor components to the greatest extentpossible in order to produce a motor which is simple and easy toassemble with a minimum of register machining on the parts to beassembled.

It is a further object of the present invention to provide a small andrelatively inexpensive electrical motor in which the end caps and rotorare machined after assembly and which can be easily and reliably cleanedof machining debris.

According to another aspect of the invention, a further object is toprovide an electrical motor which, although it has end caps and statorof substantially different coefficient of thermal expansion, prevents orminimizes walking between those elements.

These and other objects are achieved in accordance with the presentinvention by providing a motor fabrication technique and the resultingmotor which in an early stage of assembly achieves register of a pair ofend caps with a wound stator lamination stack. Such registration can beachieved, for example, by bolts passing through clearance holes in oneof the end caps, clearance holes stamped in the laminations of thelamination stack, and into an aperture in the other end cap while thoseelements are held in position on an arbor. Having achieved this initialregistration and created a unitized stator assembly, the stator assemblyis then potted, fixing the positional relationship between the end capsand the stator lamination assembly. Following potting, the central boreis machined, preferably by lapping, to form bearing mounting surfaces inthe end caps and to machine the internal bore of the stator laminationstack to provide a running air gap between the rotor and stator. A rotorassembly, comprising the rotor section itself and a pair of bearings,has the bearings of slightly larger outer diameter than the statorsection. The rotor assembly is inserted into the machined bore in thestator assembly and fixed in place to produce a completed motor.

It is a feature of the invention that the potting material fixes therelationship between the end caps and stator lamination stack, andsubsequent machining of the internal stator bore accommodates for anymisregistration which might have occurred during the initialregistration before potting.

It is a further feature of the invention that the smooth continuousinternal bore of the stator assembly prevents machining debris fromfalling into the motor and provides a stator assembly which can beeasily cleaned following machining.

It is a further feature of the invention that heat transfer from themotor is enhanced by virtue of the potting material which fills internalvoids and has a better thermal coefficient than air which normally fillssuch voids in conventional motors.

According to a further feature of the invention, the potting materialprevents walking of the metallic stator components, particularly in thecase where the end caps have significantly different thermal coefficentsof expansion than the lamination stack.

According to a optional feature of the invention, the motor can beproduced in an "enhanced" configuration, having permanent magnetsinserted between the teeth of the stator poles. The potting material isallowed to envelope the stator poles, thereby securing the permanentmagnets into the gaps between the teeth. Subsequent machining of thebore removes some of the potting material and machines sharp squarefaces on the stator teeth while still allowing the potting material toassist in holding the permanent magnets in their pole slots. Thesharpness of the stator teeth edges is enhanced by the potting materialin the gaps between the teeth which serves as a backing member for theteeth, preventing distortion which can be caused by material shearing orslight tooth deformation during machining. The magnetic debris generatedduring machining of the inserted magnets is easily and reliably cleanedfrom the smooth continuous bore of the potted stator.

Other objects and advantages will become apparent upon reference to thedetailed description when taken in conjunction with the drawings, inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating a completely assembled motorconstructed in accordance with one embodiment of the present invention;

FIG. 2 is a partly exploded view showing a motor according to theinvention with the rotor assembly removed from the stator assembly;

FIG. 3 is a process flowchart illustrating the steps of constructing amotor in accordance with the present invention;

FIG. 4 is an elevational view illustrating a partially assembled rotor;

FIG. 5 is an elevation illustrating a rotor assembly with bearings andbushings in place and ready for insertion into a stator assembly;

FIG. 6 is an elevation illustrating an individual stator lamination;

FIG. 7 is a persspective view showing an assembled lamination stack madeup of individual laminations as illustrated in FIG. 6;

FIG. 8 is a perspective view illustrating the lamination stack of FIG. 7with insulators and windings in place;

FIG. 9 illustrates a wound stator assembly with end caps and printedcircuit board in place;

FIG. 10 is a perspective view illustrating the connector end of thelamination stack of FIG. 7;

FIG. 11 is a partial view illustrating the relationship between theconnectors of FIG. 10 and the mating circuit board;

FIG. 12 is a partial view (without end cap) illustrating the partlycompleted stator assembly of FIG. 9 after potting; and

FIG. 13 is a view similar to FIG. 12 illustrating the potted statorassembly after it has been machined and is thus ready for insertion ofthe rotor assembly.

While the invention will be described in connection with certainpreferred embodiments, there is no intent to limit it to thoseembodiments. On the contrary, the intent is to cover all alternatives,modifications and equivalents included within the spirit and scope ofthe invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to drawings, FIG. 1 shows a perspective view of a hybridpermanent magnet stepping motor, and FIG. 2 a partly exploded viewshowing the rotor and rotor retaining elements removed from the statorassembly. It will be noted at the outset, however, that while theinvention will be described in connection with a hybrid stepping motor,it is also applicable to other motor types, particularly in the smallsize range. For example, a brushless variable reluctance stepping motorof the Chai et al. type could be constructed in accordance with thepresent invention by configuring the rotor and stator assembly as avariable reluctance motor. Similarly, the invention is applicable topermanent magnet brushless motor designs, switched reluctance motors,enhanced variable reluctance motors, as well as enhanced and unenhancedstepping motors of the hybrid stepping type. Finally, induction motorscould also utilize the present invention as could other motor types aswill be apparent to those skilled in the art upon reading the followingdetailed description.

Referring to FIGS. 1 and 2, there is shown a hybrid stepping motorgenerally indicated at 20 comprised of a stator assembly 21 and a rotorassembly 22. The rotor assembly is fitted with bearings 23, 24 which inturn mount in end caps 25, 26 to support the rotor assembly 22 forrotation in the stator assembly 21. The end caps 25, 26, sandwich acentral stator lamination stack 27 which carries stator poles andwindings (not shown in FIGS. 1 and 2). In the currently preferredembodiment, the end caps 25, 26, and the lamination stack 27 are securedtogether, initially by means of mounting bolts 28 which preferably passthrough clearance holes in front end cap 26 and lamination stack 27 andare secured in apertures in rear end cap 25. In a further embodiment,the bolts 28 and the end cap apertures for receiving those bolts can beeliminated (as illustrated in FIG. 1) and the end caps initially securedto the stator assembly by means of a suitable adhesive applied at theseams indicated by 28'. While that arrangement places greater emphasison the initial securement by means of adhesive, it is advantageous ineliminating the need for mechanical devices such as bolts and thenecessary apertures for providing the initial unitized assembly.

As will be described in greater detail below, the unitized elementsafter being joined by bolts, adhesive, or the like are further securedtogether by means of a potting compound which completely encapsulatesthe secured members and preferably projects partly into a central bore30 after potting. The bore 30 is then machined as by lapping to formbearing surfaces 31, 32 in the end caps 25, 26 and also to form a smoothbore 33 through the lamination stack 27, contiguous with the bearingsurfaces 31, 32. Retaining rings 35, 36 secure the rotor assembly in thestator assembly. The front end cap 26 has a flange 37 which in turn hasa machined surface 38 with a mounting boss 39 to locate the motor in amounting bracket. Mounting holes 40 provide means for mounting the motorto its bracket (not shown). The rear end cap 25 is provided with anelectrical connector 41 for supplying power to the stator windings.

As shown in FIG. 2, the rotor assembly 22 includes a rotor shaft 50which supports a rotor section 51, (i.e., the portion of the rotor whichis magnetically active) and outboard bearings 23, 24. In the illustratedembodiment, the rotor comprises toothed lamination sections 52, 53separated by a permanent magnet 54. The magnet is positioned to providethe lamination sections 52, 53 with opposite magnetic polarities, forexample, making lamination section 52 a north pole and laminationsection 53 a south pole. The laminations are formed with external teeth,of the same pitch as the teeth associated with the stator assembly. Theteeth of section 53 are offset by one half pitch with respect to theteeth of the section 52 in order to form a hybrid permanent magnetrotor. Thus, when the stator windings are energized by drive currentcoupled through connector 41, the rotating magnetic field which isproduced in the stator tends to successively align the rotor laminationsections 52, 53, with the field, causing the motor to step in sequencewith the rotor field. Control of the rotational rate and direction ofthe stator field thus allows control of the rate and direction of rotorrotation.

Turning now to FIG. 3, there is illustrated the process for fabricatinga motor in accordance with the present invention. Concentrating first onthe rotor assembly, it is seen that the primary raw materials which goto make up the rotor are brought together at process step 100, andinclude rotor shafts, rotor laminations (or prestacks) and magnets.Those items are assembled at a step 101 and the assembled rotor whichresults is best illustrated in FIG. 4. There is shown a rotor shaft 50having a pair of lamination stacks 52, 53 disposed thereon, with apermanent magnet 54 interposed between the lamination stacks forming arotor section 51 intended to be driven by the rotating magnetic fieldproduced by the stator. In a hybrid permanent magnet stepping motor, therotor laminations 52 and 53 have alternate teeth and valleys of a givenpitch corresponding to the stator pitch, and the teeth in the sections52 and 53 are offset with respect to each other by one-half pitch. Themagnet 54 serves to magnetically polarize the stacks 52, 53 with, forexample, the stack 52 being a north pole and the stack 53 being a southpole.

The shaft 50 has a pair of machined sections 60, 61 adapted to receivethe inner race of bearings for support of the rotor. The shaft 50 canhave its output end keyed as illustrated or unkeyed if desired, and themotor can also be configured with an output shaft on the rear end toform a double-ended motor. Such constructional details form no part ofthe present invention and will not be emphasized herein.

Having assembled the rotor 22 in the step 101 (FIG. 3), the rotor isthen passed to a grinding station where step 102 is performed to grindthe rotor outer diameter. Such grinding tends to produce teeth in thelamination stacks 52, 53 which have relatively sharp corners. Inaddition, the grinding step produces a rotor which is substantiallyconcentric and therefore can operate in a carefully machined stator borewith a relatively small air gap.

Having thus configured the rotor magnetic section 51, and aftermachining debris is cleaned from the rotor, a step 103 is then performedin which bearings 22, 23 are assembled onto the bearing support surfaces60, 61 of the rotor. In the exemplary embodiment, spacer bushings 62, 63are interposed between the bearings 23, 24 and the lamination stacks 52,53 respectively. The bushings ride between the lamination stack and theinner race of the bearings to form a spacer element to properly locatethe bearings on the shaft. The bearings are press fit on the shaft,preferably in an appropriate fixture, in the step 103. Referring to FIG.5, there is shown the rotor assembly including bearings 23, 24 and thespacing bushings 62, 63, providing a rotor assembly which is ready forinsertion into a stator assembly. FIG. 5 illustrates, in somewhatexaggerated fashion, the fact that the outer diameter of the bearings23, 24 is slightly greater than the outer diameter of the rotor section51. It was previously noted that the stator bore is a continuousstraight through bore formed in a single operation after assembly of thestator. Thus, providing the bearings 23, 24 with a slightly greaterouter diameter than the rotor section 51 allows the entire rotorassembly to be inserted into the bore, with the outer race of thebearing 23, 24 seating in the bearing surfaces in the end caps while therotor section 51 has a sufficient, although a very small, clearance forrotation. The precision thus achieved allows the motor to be configuredwith a relatively small air gap, thus allowing highly efficientoperation.

As illustrated in FIG. 3, the initial raw material component for thestator assembly procedure is individual stator laminations which areassembled in a step 105. An individual lamination 27a is illustrated inFIG. 6. It is seen that each lamination, which can be formed bystamping, has a series of poles 66a with a plurality of teeth 67a formedon each of the poles. In a hybrid permanent magnet stepping motor, thepitch of the teeth 67a is the same as the pitch of the rotor teeth. Thepoles 66a are separated by gaps 68a which provide an area for receivingthe stator windings. The laminations also have punched clearance holes69a through which the bolts 28 can pass for initially registering thestator assembly. Preferably a clearance hole 69a is associated with eachpole 66a such that the stator laminations are symmetrical and can beinstalled in any of eight orientations. Thus, it is possible in assemblyto turn the stator laminations with respect to each other such that thegrain of the steel from which the laminations are made is not in asingle direction, allowing the magnetic properties of the laminationstack due to grain to be averaged.

As illustrated in FIG. 7, the step 105 (FIG. 3) is implemented byassembling a stack of laminations of a predetermined height and affixingthe laminations together, in the illustrated embodiment by means ofwelds 70. Alternatively, prestacks, i.e., groups of laminations joinedby stamped dimples formed during the lamination stamping operation maybe used. Using the welding technique, preferably a stack of laminationsis placed under pressure, and automatic machinery gauges that thelamination stack is of the appropriate height before the welds are made.If it is not laminations are either added or removed until the desiredheight is obtained at which point automatic welding equipment preferablyapplies four welds 70 at corners of the lamination stack displaced 90degrees from each other. As seen in FIG. 7, the assembled laminationstack thereupon provides a pole structure 66 separated by inter-polewinding gaps 68, each pole structure having axially disposed teeth 67 ofa predetermined pitch. It is also seen that the clearance holes 69 arealigned such that an assembly bolt or potting injection needle can passthrough the lamination stack at the appropriate point in the assemblyprocess.

After the lamination stack is assembled, and in the optional case wherean "enhanced" motor is to be produced, in a step 105' elongate magneticstrips are inserted in each gap 85 between stator teeth 67 (see, forexample, FIG. 12). As will be described below, the magnets which areinserted between stator teeth tend to enhance the magnetic properties ofcertain classes of motor. The magnetic strips have sufficient frictionalengagement with and magnetic attraction for the gaps into which they areinserted to temporarily maintain the strips in place during subsequentmanufacturing steps until they are firmly secured in their gaps by meansof injected potting material.

Following the magnet insertion step 105' if performed, or the simplewelding of the lamination stack 105 for a non-enhanced motor, as shownin FIG. 3 subsequent operations are performed on the assembledlamination stack to associate the stator electrical components with thestack. In other words, the stack is insulated, wound, and the windingsare terminated. In FIG. 3 the insulation step is indicated at 106.Preferably discrete insulators are supplied along with the welded statorlamination stack in order to provide appropriate insulation. Turningbriefly to FIG. 8, one end of the insulator assembly is schematicallyillustrated at 71 and is shown to completely line the slot 68 as well asto cover the face 72 of each pole 66. Upstanding insulator sections 71'will interface with the end cap when they are juxtaposed and boltprotectors 71" insulate the through-bolts 28 and prevent contact betweenthe bolts and the windings. A mating end for the insulator isillustrated in FIG. 10 at 90. It is seen that the insulator 90 issimilar to the insulator 71 in that it provides protectors 91 for thethrough bolts, a face 92 for insulating the end of the pole, channels 93which completely line the inter-pole slots, and upstanding projections93' for interfacing with the associated end cap. In addition, theinsulator 90 includes connector means 94 for terminating the windings aswill be described below and upstanding standards 94' which serve twopurposes. First of all, they provide a seat on which the circuit boardwhich carries the connectors for mating with the connector means 94seats, and secondly they provide a path for jumper walkers connecting apair of coils, keeping the jumper wires in a controlled safe locationwhere they will not short with other elements of the motor. Moreparticularly, in many motors the automatic winding equipment winds onecoil and then is moved to another pole, carrying the wire along to winda second coil before the second end of the wire is terminated. In thatcase, the wire is brought by automatic winding equipment around theoutside of the adjacent standard 94' and the inside of the boltprotector 91 so that it is restrained to the outside of the statorassembly but assured to be free of contact with the through bolt.

Having thus insulated the pole structure, windings schematicallyillustrated at 73 are applied, preferably automatically, to each of thepoles in a step 107 (FIG. 3). It will be appreciated that in the typicalcase a winding of significant dimension (more significant thanillustrated in FIG. 8) will build up in order to get the necessary turnson each pole. It is seen, however, that the turns are insulated from themagnetic structure by means of the insulator 71. In addition, when theautomatic winding equipment winds two coils before terminating thewinding, the exposed inter-pole connection is routed between thestandards 94' and the bolt protectors 91 as described above. Theautomatic winding equipment which forms the coils 73 also preferablyautomatically terminates the coils in the connectors 94. In thepreferred embodiment, the coils are bifilar wound, and each connector 94provides for terminating two individual wires. Thus, at the start of thewinding, the automatic winding equipment lays the beginning end of acoil into slot 95, then winds two of the poles, then lays the end of thecoils in a further slot 95a in a second one of the connectors.

With all of the coils wound, and the beginning and ending ends of thecoils inserted in appropriate slots 95, 95a, a step 108 (FIG. 3) is thenperformed to terminate the windings. In the illustrated embodiment thetermination is accomplished via a printed circuit board 75 which carriesthe connector 41 and which also carries a plurality of tabs 76 connectedto appropriate pins of the connector 41 by means of printed circuitboard connections. In performing the termination step 108, as best shownin FIG. 11, the printed circuit board is put in place over theconnectors 94, and the tabs 76 forced into the connectors 94. Using theillustrated terminating means, when the tabs 76 are forced into theconnectors 94, mating apparatus on the tabs and connectors serves toforce the wire which had been layed into slots 95 into electricalcontact with the mated tabs and connectors, secures the wire in place,and severs the tail of the wire for removal. Thus, with the printedcircuit board in place, all of the terminations are automatically madeand electrical connections are available from all of the coils to theconnector 41.

It is noteworthy that eight ends of the eight coils are individuallyterminated and separately brought to associated pins in the connector41. That allows manufacture of basically the same motor while stillproviding for sale of a variety of motors with different polestructures. More particularly, in stepping motors and otherelectronically driven motors it is conventional to use four pole, sixpole or eight pole motors with drivers of related configuration. In aneight pole motor, an eight pole drive would be used to individuallycontrol each coil pair. In a four or six pole configuration, however,the eight motor poles are connected together by the user in waysdetermined by the four or six pole requirements. Alternatively, themanufacturer must wind and stock separately four, six and eight polemotors. As a feature of the preferred embodiment, having broughtconnections for each of the eight pole pairs out to connector 41, amating connector 42 is supplied with the motor which has, when it isconfigured for less than four pole operation, jumpers 42a connectingselected ones of the wiring along with connections 42b for connection tothe, in the illustrated embodiment, four pole drive. Thus, supplying themating connectors 42 with appropriate jumpers 42a allows the manufactureof only eight pole motors with a simple external connection used toconvert what had been manufactured as an eight pole motor to a four orsix pole configuration.

Having thus configured the stator electrical components including themagnetic stator lamination stack and the associated electricalcomponents, a step 110 (FIG. 3) is performed to assemble the stator,including the lamination stack 27 and the end caps 25, 26. The end caps25, 26 are unmachined castings, preferably aluminum, and have no pilottabs or registration devices. In the preferred practice of theinvention, a fixture is used to register the end caps to the statorassembly before those elements are preliminarily secured to each other.The elements can be registered either from the outer diameter of thelamination stack and end caps or from the inner diameter of the bore.Preferably, the inner diameter of the bore is used for registration, andthe end caps and lamination assembly are inserted on an arbor which maybe stepped and expandable so that when the elements are placed over thearbor, it can be expanded for a central section of slightly largerdiameter and end sections of slightly smaller diameter than the centralsection to engage respectively the ID of the lamination stack and theID's of the end caps. The arbor assures that the center line of thelaminating stack matches the center lines of the end caps. When held inthat position, the elements are preliminarily secured by four bolts 28which pass through clearance holes 80 in the front end cap 26, clearanceholes 69 through the lamination stack and are secured in apertures 81 inthe rear end caps 25. Preferably the bolts 28 are of the self-threadingtype and the apertures 81 are adapted to be threaded when the bolt isinserted and driven by means such as power screw driver. Preferably, thepower screwdriver has a torque limit which is preset so that all of thebolts are inserted with very nearly the same torque thereby to minimizeany tendency for the end caps and lamination stack to misalign.

Since, in practicing the invention, the motor is to be filled with aliquified potting compound, the practice of the invention furthercontemplates the sealing of the stator elements to prevent leakage ofthe potting material when it is in its liquid form. One potential areafor leakage is between the connector 41 and the associated end cap 25and lamination stack. In practicing this aspect of the invention, theconnector 41, the end cap 25 and the lamination stack 27 areparticularly configured with respect to each other to prevent leakage.More particularly, a three-sided tongue and groove arrangement 41a (seeFIGS. 2 and 9) is provided to seal three sides of the connector, and theconnector is provided with an abutment 41b which engages the end of thelamination stack 27 and is snugged thereagainst when the elements areassembled in the fixture. The tongue and groove arrangement 41a and theabutting seal 41b prevent leakage of potting material from around theconnector 41. In addition, leakage which could occur between the margins28' of the end caps 25, 26 is prevented by application of a sealantbetween those elements before they are inserted on the arbor forassembly. It is preferred to use a sealant sold under the trademarkLoctite, and sealant is applied to the edges of the end caps 25, 26immediately prior to insertion of those elements in their respectivepositions on the assembly arbor. A small quantity of such sealant ispreferably maintained in an annular trough and the operator simplytakes, for example, end cap 25 and inserts the open end into the troughto apply a thin layer of sealant on the edge of the end cap flangebefore placing the end cap on the arbor. Similarly, after placement ofthe lamination stack 27 over the first end cap 27 on the arbor, the endcap 26 is dipped before it is placed on the lamination stack.

As noted above, and as illustrated in the FIG. 1 embodiment, in somecircumstances it may be desirable to eliminate the connecting bolts 28together since the integrity of the final motor will be assured by theinner mass of potting material which completely fills the statorassembly. In that case, the end caps 25, 26 would be modified toeliminate the holes which accommodates the bolts 28. However, it ispreferred to retain the apertures 69 in the lamination stack since theycooperate with the epoxy injecting apertures (to be described below)which are formed in the end cap 25. When forming a non-bolted unitaryassembly, a liquid adhesive 28" is applied to the end caps 25, 26 beforeassembly on the arbor, just as the Loctite sealant was applied in thebolted arrangement. The assembly procedure is substantially the same asin connection with the bolted configuration with the exception that theadhesive which bonds the materials together must be fast-setting becausethe unitized assembly must be left on the arbor until the adhesive hasbonded the parts together. In addition, when using the non-boltedarrangement, it is preferred to use a potting material which will curerather rapidly and whose potting and cure conditions will not affect thebond produced by the adhesive which maintains the integrity of theunitized stator assembly.

In accordance with the invention, the preliminarily secured assembly,registered by means of the assembly fixture, is permanently secured inposition by potting of the stator assembly, following which theconcurrent machining of the bearing surfaces in the end caps and thestator bore provides a registered, fixed stator assembly ready toreceive its associated rotor assembly. FIG. 9 shows the stator assemblyprior to potting with the bolts 28 in place. It also shows, as was notedabove, the printed circuit board which terminates the stator windingsand brings them to the external connector 41. With the stator assemblyin the configuration illustrated in FIG. 9, or in a similarconfiguration but without bolts 28 in the non-bolted embodiment, a step111 (FIG. 3) is accomplished to pot the stator assembly. Preferablypotting is accomplished with a central arbor inserted through the boreof the stator providing a small clearance between the stator laminationsand the arbor, but closely fitting within the end caps. Preferably, thepotting arbor is made of silicone rubber or has a silicone rubberexterior to assist in insertion and removal of the arbor. Siliconerubber has a tendency to significantly expand when heated and contractwhen cooled. Thus, the arbor is inserted into the unpotted statorassembly when cold and thus easily slides inside the bore. After pottingis completed and the rotor cooled, the arbor contracts sufficiently sothat it can be removed without the use of an arbor press.

In order to minimize voids in the potted assembly, prior to potting boththe unpotted stator assembly (with inserted arbor) and the pottingmaterial are heated. Recalling that the potting arbor expands uponheating, and that the unpotted stator assembly has two different innerdiameters, i.e., a smaller ID at the end caps and a slightly larger IDat the lamination stack, the potting arbor will expand until it securelycontacts the ID's of the end caps but leaves a small gap, in oneembodiment on the order of 0.005" between the outer surface of the arborand the stator teeth in the lamination stack. Thus, when pottingmaterial is injected into the stator assembly, a continuous runner ofpotting material will cover the stator teeth, assuring that theinter-tooth gaps are all filled with potting material.

In the preferred practice of the invention, for the purpose of potting,a pair of small apertures are provided in the rear end plate 25 whichalign with a pair of the lamination stack apertures 69. A needle isinserted through one of the end cap apertures, and extends through thestator assembly to the front end cap. Heated potting material is theninjected through the needle such that stator fills from the bottom up.Air is thus forced out of the assembly through the second end cap holeas potting material fills the voids, and the arbor prevents pottingmaterial from entering the internal bore except for the thin runnerwhich covers the stator teeth and fills the inter-tooth gaps. Pottingmaterial 82 thus completely fills all voids, forming a pair of donutswithin each of the end caps joined by finger-like elements which filleach of the winding slots 68 in the stator. After the potting materialis injected to completely fill the stator, the needle is withdrawn. Thestator is then put into a post-cure process which raises the temperatureof the assembly to about 250° F. The post-cure treatment tends tofurther stabilize the epoxy, preventing outgassing or void formationduring use of the motor, and providing an even more stable assembly.

After potting is completed and the stator assembly removed from thearbor, the stator appears as suggested in FIG. 12. As noted above, whenthe assembly with inserted arbor cools, the shrinkage of the arborallows easy removal thereof without the need for an arbor press. Thepotting material 82 completely covers all of the pole teeth 67, fillsall of the voids around the windings as shown in FIGS. 12 and 13 as wellas all voids in the end caps, to form a continuous surface extendingthrough the center of the stator assembly from end cap to end cap. Thepotting compound is preferably chosen to have a thermal coefficient ofexpansion and elasticity which is intermediate those of the end capswhich are preferably aluminum, and the stator laminations which aresteel. It will be appreciated that the aluminum end caps have a thermalcoefficient of expansion which is substantially greater than that of thestator laminations, and in normal practice would tend to "walk" as themotor heats and cools during periods of use and rest.

The shape assumed by the injected potting material within the motor issignificant. As noted above, the potting material forms a unitarystructure comprising a pair of donuts within the respective end capsjoined by, in the illustrated embodiment, eight spring-like fingersfilling the winding slots 68. Although the aluminum end caps tend toexpand more than the stator lamination stack when heated, the springfingers connected to the donuts act like a homogeneous spring to allowall of the elements to expand together (although at different rates)and, when the elements cool and contract to bring them back to theirinitial position. A presently preferred potting compound is produced byHardman Inc. (Epocap 16358) which is a heat cure epoxy potting andencapsulating compound having a substantial quantity of an appropriatefiller.

Furthermore, the potting material is selected to have a thermal heattransfer constant which is substantially better than that of air, andthus the motor is better able to conduct heat generated in the stator tothe outside to dissipate the heat. As a result, temperature buildup inthe motor is reduced, particularly as compared to a motor having aconventional case, and efficiency in terms of watts input per unittorque output, is substantially enhanced.

Having thus potted the stator assembly in the step 111 (FIG. 3) a step112 is performed to machine the central bore. Any number of machiningtechniques can be utilized to form the central bore including boring,grinding, broaching, honing or lapping. In the presently preferredembodiment of the invention, lapping is used preferably with multiplelapping stations. In very high volume manufacture broaching may bepreferable. In the currently preferred practice of the invention,multiple diamond lapping stations are used, and are arranged to assurethat some material is removed at each station. The initial cuts removeprimarily potting material from the stator and aluminum from the endcaps until the lapping operation has opened the bore 33 so that thelapping tools contact the stator laminations. Thus, as the successivelapping opertions are carried out, material is ultimately removed fromthe teeth 72 of the stator assembly and at the same time from thebearing surfaces 31, 32 of the end caps to machine a smooth andcontinuous bore 33 through the entire stator assembly. The machining inthe end caps forms bearing surfaces for slip fit of the rotor bearingswhile, as shown in FIG. 11, the machining of the lamination stacksharpens the corners of the stator pole teeth 67 to produce a uniformbore for minimum air gap and enhanced magnetic coupling.

It will be appreciated that in practicing the invention no machining wasneeded on either of the end caps 25, 26 prior to the lapping operationwhich forms the bearing surfaces. Typically, motors have machinedflanges for accurate mounting of the motor in its end use apparatus.Thus, having completed the lapping operation and thus formed a centralbore in the motor which will define the center line of rotation of therotor, that bore is used as a registration pilot for machining amounting flange 38 on the face of the motor. Typically, the flange ismachined on a lathe, and subsequent to machining of the mounting flange,the tool is changed and grooves 83 are machined in the bearing surfaces31, 32 for accepting the retaining rings 35, 36. Following theperformance of the step 113 for such finish machining, the statorassembly is prepared to receive a rotor and thus produce an assembledmotor. More particularly, a step 115 is performed in which the rotorassembly produced in the step 103 is joined with the stator assemblyproduced in the step 113 to produce a completed motor. It is simplynecessary to install one of the retaining rings, say, front ring 36 inthe end cap 26, than to slide the rotor assembly with bearings into thebore 33. Thrust means 36a such as a wave washer and spacers 36b, ifnecessary, are then installed above the bearing to load the bearings inone direction. Following insertion of the rotor, spacers, if any, andloading means, the rear clip 36 is put in place producing a completedmotor. A step 116 (FIG. 3) is then performed to magnetize the rotor in aconventional fashion.

One of the significant advantages which is achieved by the assemblytechniques of the present invention is the ability to produce "enhanced"stepping motors, that is, motors having magnetic inserts between thestator teeth for enhancing the flux paths and producing correspondinglyenhanced performance. Reference is made to the following U.S. patentsfor a description of the structure and function of enhancement achievedby insertion of magnets in the slots between the teeth of the polestructure: Horber U.S. Pat. No. 4,712,028, Mastromattei U.S. Pat. No.4,713,570 and Gamble U.S. Pat. No. 4,763,034. FIGS. 12-13 of the presentapplication show the techniques of the aforementioned patents applied tothe instant motor structure. No attempt was made to illustrate theinserts in the other smaller scale figures in order to avoid confusingthose drawings. However, from the illustration of FIGS. 12 and 13, itwill now be apparent to those skilled in this art how the enhancementmagnets are utilized in practicing the present invention. As shown inFIGS. 12 and 13, magnetic segments 85 are inserted in gaps 86 betweenthe stator teeth 67. While in the past it had been necessary to glue thesegments in place then vacuum impregnate the pole faces, or to varnishthe stator, such steps can be avoided in practicing the presentinvention. More particularly, the magnetic segments 85 are preferablyrelatively strong magnets such as samarium cobalt. Since the internaldiameter of the stator must be honed prior to insertion of the rotor, itwill be appreciated that magnetic debris is generated as the magneticsegments are honed. This debris is highly magnetic and tends to adhereto the laminations and the like and, in the past, was very difficult toremove.

In practicing the present invention, the potting material is allowed toenter the internal bore (by means such as the aforementioned pottingarbor) and thus entirely encompasses the magnetic segments 85. Acompletely smooth bore is provided which remains smooth as it is lappedor honed. Thus, even though highly adherent magnet debris is generatedin the honing operation, since the internal bore 33 is completelysmooth, it is a relatively easy matter to mechanically clean the borewith an appropriately sized cleaning tool and solvent or a high pressurewash to reliably remove even the most adherent magnetic particles, andprovide a clean and unobstructed bore in which the rotor can rotate. Inaddition, the windings are completely potted so there is no danger ofshorting from machining debris and also the electrical terminations forthe windings are potted to produce a completely sealed unit which themachining debris never enters.

FIGS. 12 and 13 illustrate a further benefit of the potting andsubsequent honing procedure when used with motors of the enhanced type.It is recalled that the inner diameter of the bearing surfaces at theend caps before honing is slightly smaller than the inner diameter ofthe stator lamination stack, and that potting with a straight arbor suchas the aforementioned silicone rubber potting arbor assures a thin (suchas 0.005") runner of potting material covers all of the teeth. In viewof the fact that potting is accomplished from the bottom up andtherefore reasonable pressure is generated during the potting operation,it will be appreciated that potting material is forced into any crevicein the inter-tooth gaps, serving as a bonding agent to hold theenhancement magnets in place. Subsequent lapping of the potted statorproduces a smooth bore in which the stator teeth and magnets areexposed, potting material also forms a part of that smooth bore andassures the holding of the enhancement magnets in place in theinter-tooth gaps.

The potting material between the teeth also serves an important purposefor non-enhanced motors. In most machining operations, including thepreferred diamond lapping operation, material is removed from the statorteeth by a tool which is constantly moving in the same direction, oftenat a relatively high rate. It is noted that using the preferred diamondlapping technique, it is preferred to utilize a tool speed of about 650rpm and an advance rate of about 20" per minute. With no backingmaterial between the teeth, particularly with high speed cutting, theshearing effect of the tool against the unsupported tooth would tend todistort the tooth trailing edges as material is sheared from the toothface. Potting of the stator in such a way that all of the inter-toothgaps are filled prior to machining puts a layer of backing material inthe form of the potting material between the teeth such that as the toolremoves material from the tooth it is supported by the backing materialin the gap and does not distort or shear in such a way as to deform theedge as the tooth is machined. The result is very sharp edges on theteeth substantially free of distortion, and such sharp teeth contributeto the uniformity of magnetic paths through the motor.

FIGS. 12 and 13 also illustrate the thin runner of material formed overthe stator teeth during the potting operation and later machined away.More particularly, referring to FIG. 12, it is seen that pottingmaterial completely fills the inter-tooth gap including all intersticesbetween the magnets 85 and the teeth 67, covering all of the magnets andteeth to form a thin layer of potting material comprising a continuoussurface throughout the bore of the motor. FIG. 12 illustrates the motorafter potting, after removal of the arbor, and prior to lapping of thebore. FIG. 13 illustrates the condition after the bore is lapped showinga continuous surface for the internal bore, such continuous surface atdifferent parts including the potting material, exposed teeth 67 andexposed magnets 85 (when present).

It will thus be appreciated that what has been provided is a new andreliably assembled electrical motor in which the stator is assembledfrom substantially unmachined parts. After a lamination stack isassembled, insulated, wound and terminated, a pair of unmachined endcaps are disposed at either end of the lamination stack and initiallysecured in register to form a unitary assembly. The preferred forms ofsecuring initial register are by through bolts or quick cure adhesive.Once the assembly is unitized, it is then potted, permanently locking inthe register which had been initially secured. Having thus potted theentire stator assembly, the internal bore is then lapped to produce astraight, smooth uninterrupted internal bore which forms bearingsurfaces in the end caps and the bore through the lamination stack forreceiving the rotor. A previously assembled rotor assembly including aground lamination stack of a given outer diameter and assembled bearingsof slightly larger outer diameter is then inserted into the smooth boreof the stator assembly, secured in place, and the motor, aftermagnetization of the rotor, is ready for service.

What is claimed is:
 1. A method of producing an electrical motorcomprising the steps of:assembling a rotor assembly on a rotor shafthaving a rotor section of predetermined diameter and bearings on therotor shaft of slightly larger diameter than the rotor section;assembling a plurality of stator laminations, stator windings,terminations and stator end caps to form an assembled but uncompletedstator assembly; potting the assembled stator assembly to form aunitized potted stator assembly; machining a continuous bore through thecenter of the potted stator assembly to produce a concentric bore forthe rotor section and mounting surfaces for the rotor bearings; andmounting the rotor assembly into the stator assembly, the mounting stepincluding inserting the rotor assembly into the bore of the statorassembly with the bearings engaging the bearing mounting surfacesmachined in the end caps during the bore forming step.
 2. The method ofclaim 1 in which the assembling step comprises attaching a plurality ofstator laminations together to form a lamination stack having clearanceholes therethrough, and assembling the stator end caps and laminationstack by means of bolts passed through said clearance holes andclearance holes in at least one of the end caps to form the assembledbut uncompleted stator assembly.
 3. The method of claim 2 in which thestator laminations are attached by welding.
 4. The method of claim 1 inwhich the assembling step comprises attaching a plurality of statorlaminations together to form a lamination stack, and assembling thestator end caps and lamination stack by means of adhesive attaching theend caps to the lamination stack to form the assembled but uncompletedstator assembly.
 5. The method of claim 2 in which the end caps areunmachined prior to assembly.
 6. The method of claim 1 in which thepotting step comprises injecting potting material into the end caps andstator laminations to form a smooth continuous bore through the statorassembly having all voids therein substantially filled with pottingmaterial.
 7. The method of claim 6 in which one of the end caps has apair of apertures registered with apertures through the laminations, andthe potting step comprises inserting a needle through one of the end capapertures and through the stator lamination stack, injecting pottingmaterial into the assembled stator assembly through the needle therebyto fill the stator assembly from the bottom up, and expelling airthrough the second end cap aperture as the assembled stator assemblyfills with potting material.
 8. The method of claim 7 in which thepotting step further comprises inserting an arbor through the bore ofthe assembled but uncompleted stator assembly prior to potting saidarbor engaging the end caps but being out of contact with thelaminations, and the injecting of potting material serving to forcepotting material between the arbor and the stator laminations to coverthe inner surface of the laminations and form said smooth continuousbore.
 9. The method of claim 8 wherein the potting step furthercomprises heating the assembled but uncompleted stator assembly afterinsertion of the arbor to a degree sufficient to expand the arbor intocontact with the inner surfaces of the end caps.
 10. The method of claim1 in which the stator laminations have a plurality of inwardly facingstator poles, each of the poles having a plurality of pole teethseparated by gaps, and in which the potting step comprises potting thestator assembly in such a way as to dispose potting material in the gapsbetween the stator teeth, and the bore forming step comprises machininga bore through the stator by removing both potting material and materialfrom the stator teeth while forming said central bore.
 11. The method ofclaim 10 in which the potting step further comprises inserting an arborthrough a central opening in the end caps and laminations, and heatingthe assembled but uncompleted stator assembly with inserted arbor to adegree sufficient to cause said arbor to expand into contact with theinner surfaces of the end caps but leaving a gap between the arbor andthe pole teeth, and injecting potting material into the stator assemblywith inserted arbor so as to force potting material between the arborand the pole teeth, thereby to fill all the gaps between the pole teethwith potting material.
 12. The method of claim 11 in which one of theend caps has a pair of apertures registered with apertures through thelaminations, and the potting step further comprises inserting a needlethrough one of the end cap apertures and through the apertures in thelaminations, injecting potting material through the needle to fill thestator assembly with potting material from the bottom up while forcingair out of the second end cap aperture.
 13. The method of claim 12 inwhich the bore forming step comprises lapping said bore to form thecentral bore for the rotor section and mounting surfaces for the rotorbearings.
 14. The method of claim 13 in which the lapping step comprisesdiamond lapping.
 15. The method of claim 12 in which the bore formingstep comprises honing said bore to form the central bore for the rotorsection and mounting surfaces for the rotor bearings.
 16. The method ofclaim 12 in which the bore forming step comprises broaching said bore toform the central bore for the rotor section and mounting surfaces forthe rotor bearings.
 17. The method of claim 12 further includinginstalling a connector in one of the end caps for terminating the statorwindings in such a way as to prevent leakage of potting material in thearea of the connector during the potting step.
 18. The method of claim12 further comprising the step of installing permanent magnets on thestator poles in the gaps between the stator teeth, and the potting stepcomprises covering the installed permanent magnets with potting materialto assist in holding the magnets in place between the stator teeth. 19.The method of claim 18 in which the bore forming step comprises diamondlapping of the internal bore to remove potting material leaving exposedstator teeth and inserted permanent magnets held in place in the gaps bypotting material.
 20. The method of claim 1 in which the electricalmotor is a hybrid permanent magnet stepping motor and in which the stepof assembling the rotor comprises assembling at least two stacks oftoothed rotor laminations separated by a permanent magnet with the teethof the respective rotor lamination stacks being offset from each other.21. A method of manufacturing an electrical motor comprising the stepsof:producing a rotor assembly including a rotor shaft carrying aplurality of rotor laminations stacked to form a rotor section, andassembling rotor bearings on the rotor shaft, the rotor bearings havingan outer diameter which is slightly larger than the outer diameter ofthe rotor section; assembling a stator lamination stack by fixedlyjoining a plurality of stator laminations, the stator lamination stackhaving stator poles on the inside and an outer surface on the outside;forming a wound stator by assembling insulators and windings on thepoles in the stator lamination stack; forming a capped stator assemblyby assembling stator electrical termination means and a pair of end capson the wound stator assembly, the end caps being assembled leavingexposed the outer surface of the stator lamination stack between the endcaps; potting the capped stator assembly to form a potted statorassembly; machining a bore in the center of the potted stator assemblythrough the end caps and the stator lamination stack, the bore throughthe stator lamination stack forming a bore for rotation of the rotorsection, and the bore through the end caps forming mounting surfaces forthe rotor bearings; and inserting the rotor assembly into the bore ofthe stator assembly with the bearings at either end of the rotorassembly engaging the bearing mounting surfaces in the end caps with therotor section intermediate, the mounting step including fixing thebearings in place in their respective end caps.
 22. The method of claim21 in which the assembling step comprises welding a plurality of statorlaminations together to form a lamination stack having clearance holestherethrough, and assembling the stator end caps and lamination stack bymeans of bolts passed through said clearance holes and clearance holesin at least one of the end caps to form a loosely registered cappedstator assembly.
 23. The method of claim 22 in which the end caps areunmachined prior to assembly.
 24. The method of claim 21 in which thepotting step comprises injecting potting compound into the end caps andstator laminations to form a bore through the stator assembly which iscompletely sealed by potting material.
 25. The method of claim 21 inwhich the stator laminations have a plurality of inwardly facing statorpoles, each of the poles having a plurality of pole teeth, and in whichthe potting step comprises potting the capped stator assembly in such away as to dispose potting material in gaps between the stator teeth, andthe machining step comprises machining a bore through the potted statorassembly by removing both potting material and material from the statorteeth while forming said central bore.
 26. The method of claim 25 inwhich the machining step comprises lapping said bore to form the centralbore for the rotor section and mounting surfaces for the rotor bearings.27. The method of claim 26 in which the lapping step comprises diamondlapping.
 28. The method of claim 25 further comprising the step ofinstalling permanent magnets on the stator poles between the statorteeth, and the potting step comprises covering the installed permanentmagnets with potting material to assist in holding the magnets in placebetween the stator teeth.
 29. The method of claim 23 in which themachining step comprises diamond lapping of the internal bore to removepotting material leaving exposed stator teeth and inserted permanentmagnets.
 30. The method of claim 21 in which the electrical motor is ahybrid permanent magnet stepping motor and in which the step ofassembling the rotor comprises assembling at least two stacks of toothedrotor laminations separated by a permanent magnet with the teeth of therespective rotor lamination stacks being offset from each other.